netdev - Re: tbench wrt. loopback TSO

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Date:	Mon, 27 Oct 2008 11:39:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	zbr@...emap.net
Cc:	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org, efault@....de,
	mingo@...e.hu, a.p.zijlstra@...llo.nl, herbert@...dor.apana.org.au
Subject: Re: tbench wrt. loopback TSO

From: Evgeniy Polyakov <zbr@...emap.net>
Date: Mon, 27 Oct 2008 20:03:14 +0300

> On Mon, Oct 27, 2008 at 05:19:17PM +0200, Ilpo Järvinen (ilpo.jarvinen@...sinki.fi) wrote:
> > Noticed that tcp_current_mss does modulo with tso and that it is 
> > being called from a non-trivial number of places, though ovbiously only 
> > part of them should be relevant here. ...I'm not sure if that can show 
> > up though.
> 
> That's why when I see modulo something screams inside me...
> I used following patch (without goto in the tcp_tso_should_defer():

Indeed, those modulos are the worst part about the TSO code.

I'll see what I can do to get rid of them, I hate them too. :-)

One idea immediately occurs to me.  Since we're effectively limited
to a 64K TSO frame, and the MSS is some value smaller than that, we
can probably get away with a reciprocol divide.  Even using a 16-bit
inverse value would suffice, so we wouldn't need to use u64's like
some other pieces of code do.  A u32 would be enough.

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